Silver sulfide nanoparticles (Ag₂S-NPs) are taken up by plants and are phytotoxic

Abstract

Silver nanoparticles (NPs) are used in more consumer products than any other nanomaterial and their release into the environment is unavoidable. Of primary concern is the wastewater stream in which most silver NPs are transformed to silver sulfide NPs (Ag₂S-NPs) before being applied to agricultural soils within biosolids. While Ag₂S-NPs are assumed to be biologically inert, nothing is known of their effects on terrestrial plants. The phytotoxicity of Ag and its accumulation was examined in short-term (24 h) and longer-term (2-week) solution culture experiments with cowpea (Vigna unguiculata L. Walp.) and wheat (Triticum aestivum L.) exposed to Ag₂S-NPs (0–20 mg Ag L⁻¹), metallic Ag-NPs (0–1.6 mg Ag L⁻¹), or ionic Ag (AgNO₃; 0–0.086 mg Ag L⁻¹). Although not inducing any effects during 24-h exposure, Ag₂S-NPs reduced growth by up to 52% over a 2-week period. This toxicity did not result from their dissolution and release of toxic Ag⁺ in the rooting medium, with soluble Ag concentrations remaining below 0.001 mg Ag L⁻¹. Rather, Ag accumulated as Ag₂S in the root and shoot tissues when plants were exposed to Ag₂S-NPs, consistent with their direct uptake. Importantly, this differed from the form of Ag present in tissues of plants exposed to AgNO₃. For the first time, our findings have shown that Ag₂S-NPs exert toxic effects through their direct accumulation in terrestrial plant tissues. These findings need to be considered to ensure high yield of food crops, and to avoid increasing Ag in the food chain.

• Cowpea
• silver nanoparticles
• transformation
• uptake
• wheat

Acknowledgements

Support and assistance by XAS Beamline staff members C. Glover and B. Johannessen is gratefully acknowledged.

Declaration of interest

Support was provided to P. Wang as a recipient of an Australian Research Council (ARC) Discovery Early Career Researcher Award (DE130100943) and to P. Kopittke (FT120100277) and E. Lombi (FT100100337) as recipients of ARC Future Fellowships. The ARC provided further support through DP120101115 and LP100100800. Parts of this research (AS141/XAS/7040r) were carried out on the XAS Beamline at the Australian Synchrotron, Clayton, Victoria, Australia. The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the article.

Supplementary material available online

Supplementary Figures S1–S14 and Table S1–S4.